What is the basis of judgment for selecting 1A or 5A secondary current of CT in substation? How to judge the number of protection level coils?

Currently known choice is: when the transmission distance between the transformer and the detection device (wire) more than 5 meters, usually choose 1A secondary current, otherwise choose 5A current. The reason is that the transmission distance caused by the measurement error, the cause of this error many also have a lot of research to verify. Of course, but also with the actual use of the protection device required to support the detection needs of the relevant, but mainly not power-related but the detection device built-in model conversion circuit structure. For 110 or 220 substations, where the transformer before the switchgear other than all the ancillary equipment are all secondary equipment, corresponding to the primary equipment is the strong part of the power, while the transformer is all the back of the weak system. For 10kV substations, all equipment greater than or equal to the 3kV rated voltage are divided into the strong part, and the rest are all weak systems. This is different from the working environment where everyone is differently targeted and differentiated. The number of coils chosen for protection class CTs is entirely dependent on the designer and the level of protection required by the actual line protection lock, if space allows, up to 6 windings of CTs can be present (secondary side). In addition to the type of protection you said, the specific type of protection is a lot of, the classification method is not consistent - it can be the control method can also be the control of the occasion, the demand for environmental and so on, you can check the network "power system protection" to read more knowledge! ......